CN111417788A

CN111417788A - Bearing retainer limiter

Info

Publication number: CN111417788A
Application number: CN201880079065.6A
Authority: CN
Inventors: P.米尔斯; A.W.斯内尔; R.G.霍尔勒
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2017-10-05
Filing date: 2018-10-05
Publication date: 2020-07-14
Anticipated expiration: 2038-10-05
Also published as: EP3692271A1; CN111417788B; GB201716324D0; JP2020536211A; EP3692271B1; US11506212B2; US20200248701A1; JP7288436B2; WO2019069086A1; GB2567202A

Abstract

A bearing cage retainer (11) for a rolling element rotor bearing in a vacuum pump. The bearing cage retainer is configured with: an operating arrangement in which the bearing cage retainer is not engaged with the bearing cage (7) at the maximum longitudinal axial displacement limit of the outer race (5) in the direction of the retainer; and a failure configuration in which the bearing cage is misaligned due to longitudinal axial displacement of the bearing cage relative to the outer race in the direction of the retainer, and in which the bearing cage retainer engages the bearing cage and the bearing cage maintains the rolling elements spaced within the bearing.

Description

Bearing retainer limiter

Technical Field

The present invention relates to a bearing cage retainer (retainer), and in particular to a bearing cage retainer for a rolling element rotor bearing in a vacuum pump, in particular a turbomolecular pump. The invention also relates to a turbomolecular pump comprising such a stopper, a bearing system for a turbomolecular pump, and a method for assembling a turbomolecular pump.

Background

Vacuum pumps typically comprise an impeller in the form of a rotor mounted on a shaft that rotates relative to a surrounding stator. The shaft may be supported by a bearing arrangement (arrangement) comprising two bearings located at or intermediate respective ends of the shaft. Alternatively, the shaft may be a cantilever supported with two bearings at or proximate to one end of the shaft. In both arrangements, one or both of the bearings may be in the form of rolling element bearings. For example, the upper bearing may be a magnetic bearing and the lower bearing may be a rolling element bearing.

Referring to fig. 1, a typical rolling bearing (1) includes: an inner race (2) fixed relative to a shaft (3) of a vacuum pump (4); an outer race (5); and a plurality of rolling elements (6) supported by the cage (7) for allowing relative rotation of the inner race (2) and the outer race (5). Typically, the outer race (5) is fixedly attached to a bearing support damper (8), which in turn is fixedly attached to the housing (9) of the vacuum pump. The bearing support damper (8) is typically held in place by a bearing support nut (10).

The rolling element bearing (1) is lubricated to establish a carrier film spacing the bearing components in rolling and sliding contact, thereby minimizing friction and wear.

In normal use, the various parts of the bearing (1) will remain in substantially the same plane with the inner race (2), the rolling elements (6) and the cage (7), rotating about the axis (a) of the shaft (3). However, turbomolecular pumps are likely to fail due to contamination in the mechanical bearings, causing the bearing cage to shift axially and back out during operation.

If the bearing retainer is disengaged while the pump is running, the pump may fail in one of three modes.

In the first failure mode, the bearing can become noisy. Typically, during such failure modes, the user will turn off the pump and then investigate the cause of the failure. The failure mode is considered benign. The pump may be repaired.

In the second failure mode, the pump will continue to run, but will not restart once stopped. Upon restart failure, the user will typically investigate the cause of the failure. Likewise, the failure mode is considered benign. The pump may be repaired.

In the third failure mode, the bearing fails and thus the rotor strikes the stator causing the blade to break. Such failure modes are potentially dangerous and costly. Often, the pump cannot be repaired and the user's remaining equipment may become contaminated.

The present invention addresses these and other problems of the prior art.

Disclosure of Invention

Accordingly, in a first aspect, the present invention provides a bearing cage retainer for a rolling element rotor bearing in a vacuum pump (typically a turbomolecular pump).

The bearing may include an outer race, an inner race, and a plurality of spaced rolling elements within a rotatable bearing cage. Typically, the rolling elements are balls. Preferably, the bearing comprises from about 6 to about 12 balls, of which 7 are examples. Typically, the bearing cage is toothed. Typically, the rolling elements are substantially evenly spaced. Preferably, the bearing cage has an annular (ring-like) cross-section and the rolling elements are held within the ring with equidistant spacing between adjacent rolling elements.

Typically, vacuum pumps are configured to limit longitudinal axial displacement of the bearings (particularly the outer races of the bearings). By longitudinal axial displacement is meant a displacement in a direction parallel to the axis of rotation of the bearing. The longitudinal axial displacement may be limited by a removable fixture (such as a fixture nut or a bearing damper fixture nut) coupled to the outer race. The retaining nut and the bearing cage retainer may be a single, unitary structure.

The bearing cage retainer may be configured to have an operational arrangement in which the retainer does not engage the bearing cage. Preferably, the bearing cage retainer is configured to have an operating arrangement in which the bearing cage retainer is not engaged with the bearing cage at the maximum longitudinal axial displacement limit of the outer race in the direction of the retainer. Thus, the bearing cage retainer may not engage the bearing cage during normal use of the vacuum pump.

The bearing cage retainer may also be configured to have a failure configuration in which the retainer engages the bearing cage. Preferably, the failure configuration is characterized by: the bearing cage is misaligned due to longitudinal axial displacement of the bearing cage relative to the outer race in the direction of the retainer, and in a failed configuration, the bearing cage retainer is engaged by the bearing cage.

Typically, in the failure configuration, the bearing cage retainer enables the bearing cage to maintain the rolling elements spaced apart. Preferably, the bearing cage may remain rotatable in the failed configuration, preferably wherein the axis of rotation of the bearing cage is substantially coaxial with the axis of rotation of the rotor shaft and/or the inner race of the vacuum pump. Preferably, the bearing cage and/or the rolling elements are at least partially retained within the bearing (i.e., between the inner and outer races) when in the failed configuration. The bearing cage may rotate in the operating arrangement and remain rotating in the failure configuration if the turbomolecular pump is in use.

Under normal operating conditions, the bearing cage is held in a substantially axially fixed position relative to the outer race while rotating relative to the outer race. Typically, the failure configuration is characterized by an undesirable axial displacement (or misalignment) of the bearing cage relative to the outer race, which is typically a downward axial displacement. The axial displacement is relative to the position of the outer race in its normal operating configuration. The bearing cage may also be axially displaced relative to the inner race when in a failed configuration.

In an embodiment, the failure configuration provides an audible signal indicating a bearing failure when the turbomolecular pump is in use. Typically, the user will hear the audible signal and stop the turbomolecular pump. Additionally or alternatively, there may be a change in the pump vibration, such as a change in amplitude or frequency, typically an increase in amplitude. Again, the user may detect the change in vibration and stop the pump. Vibration detection may be by user inspection or by automated means, such as using an accelerometer (e.g., a piezoelectric transducer). The vibration level may be compared to a historical baseline value.

Additionally or alternatively, in an aspect, the present invention provides a bearing cage retainer for a vacuum pump comprising a rolling element rotor bearing comprising an outer race, an inner race, and a plurality of rolling elements located within the bearing cage, wherein the bearing cage retainer is configured to selectively engage the bearing cage only when in a failure configuration characterized by an audible signal indicating a bearing failure and the bearing cage remains rotatable. Preferably, the retainer is configured such that, when in the failed configuration, the bearing cage is at least partially retained within the bearing. If the turbomolecular pump is in use, the bearing holder may rotate and remain rotating in the failure configuration.

Again, the failure configuration may be characterized by axial displacement of the bearing cage relative to the outer race. Typically, the vacuum pump is a turbomolecular pump.

Additionally or alternatively, in one aspect, the invention provides a bearing cage retainer for a vacuum pump comprising a rolling element rotor bearing comprising an outer race, an inner race, and a plurality of rolling elements within a rotatable bearing cage, wherein the bearing cage retainer is configured to engage the bearing cage only when in a failure configuration characterized by axial displacement of the bearing cage relative to the outer race and maintained spacing of the rolling elements, preferably maintaining substantially uniform spacing of the rolling elements (i.e. adjacent rolling elements are all equally spaced).

Preferably, the bearing holder remains rotatable. Preferably, the bearing cage is at least partially retained within the bearing when in the failure configuration. The bearing holder may be rotating and remain rotating if the turbomolecular pump is in use.

Again, the failure configuration may be characterized by axial displacement of the bearing cage relative to the outer race.

In all aspects and embodiments, the retainer may be configured such that when the bearing cage retainer is in a failed configuration, the circumferential spacing of the rolling elements is maintained. Preferably, the rolling elements remain substantially circumferentially equally spaced. Preferably, the rolling elements remain at least partially retained in the bearing cage in the failure configuration. Preferably, at least half of each rolling element is retained within the bearing cage. Preferably, the rolling elements remain engaged with the inner and outer races. Preferably, the rolling elements are rollable in the failure configuration.

Preferably, the rolling elements are balls and in the failure configuration at least a hemisphere of each of the balls remains located between the inner and outer races of the bearing.

Thus, in a turbomolecular pump comprising a bearing cage retainer according to the present invention, if the bearing cage is displaced axially, e.g. due to contamination in the bearing, the bearing cage may engage the bearing cage retainer and be maintained in a position in which the rolling elements are retained within the bearing cage and substantially equally spaced. Although the bearings will fail, the turbomolecular pump can be safely shut down, avoiding catastrophic failure of the pump.

A bearing cage retainer according to any aspect of the present invention may include at least one bearing cage braking surface for frictional sliding engagement with a bearing cage in a failure configuration. Although not limited to any particular shape, typically, at least one braking surface is substantially annular or partially annular (i.e., a portion of an annulus). Two, three, four or five part-annular surfaces are preferred.

Typically, the ratio of the extent of the braking surface in the radial direction to the extent of the bearing cage in the radial direction is from about 1:1 to about 1:2, preferably from about 4:5 to about 2: 3.

Additionally or alternatively, the bearing cage may be configured such that in the failure configuration, the at least one braking surface only engages the bearing cage. Preferably, the braking surface does not engage the outer race and/or the inner race when in the failure configuration.

Typically, in the operating arrangement, the at least one braking surface lies in a plane intersecting the outer race and/or the inner race of the bearing, preferably the plane is substantially tangential to the axis of rotation of the bearing and/or the axis of rotation of the impeller.

Typically, the braking surface is located on the bearing cage-side surface of the annular or partially annular boss, forming part of the bearing cage retainer.

The bearing cage retainer may be configured to engage and retain the bearing cage when the turbomolecular pump is operating at full speed. Typically, at full speed, the impeller shaft and inner race will rotate at a speed greater than 50000 RPM (typically at least 60000 RPM). When the turbomolecular pump is running at full speed, the bearing cage will typically rotate at approximately 1/3 (e.g., 16000 RPM or greater, or 20000 RPM or greater) of the inner race speed.

Typically, in the operating arrangement (i.e. under normal working conditions), the bearing cage retainer does not engage the bearing cage. Additionally or alternatively, in the operational arrangement, the bearing cage retainer does not engage the bearing at all.

Typically, the outer race is rotationally fixed, while the inner race, the bearing cage and the rolling elements are rotatable substantially about an axis (i.e., they are substantially coaxial), typically they are coaxial with the axis of the rotor shaft of the turbomolecular pump. Usually, the rolling elements rotate about their own axis as well as about the axis of the rotor shaft. Typically, the inner race is coupled to the rotor shaft of the turbomolecular pump. In use, the bearing cage retainer may be rotatable relative to the outer race or fixed relative to the outer race.

The bearing cage retainer may be made of any suitable material, such as an alloy, polymer, ceramic, or composite material. Typically, the material and/or geometry of the limiter will be selected to be sufficiently stiff so as to substantially limit axial deflection of the limiter when engaged in the event of a bearing failure. The material properties may be selected such that it is suitable for the operating temperature of the bearing, for example between about 90 ℃ and about 150 ℃.

Additionally or alternatively, the material may be selected such that it may be machined to the required tolerances, thermally stable at operating and storage temperatures, and stable when exposed to any lubricant present.

Polymers suitable for use in the bearing cage retainer of the present invention may be selected from the group consisting of elastomers, thermoplastics, or thermosets. Thermoplastic materials are preferred. Typically, the polymer is selected from the group consisting of: polyolefins such as polyethylene and polypropylene; polyvinyl chloride, polyethylene terephthalate; and fluoropolymers such as polytetrafluoroethylene, and derivatives and copolymers thereof. High performance thermoplastics may also be used. Preferred high performance thermoplastics may be selected from the group consisting of liquid crystalline polymers including aromatic polyamides and aromatic polyesters, aromatic polyimides, polyamides, polysulfones, polyethyleneimines and Polyetheretherketones (PEEK), or derivatives or copolymers thereof.

The polymer may additionally comprise one or more from the group consisting of: antistatic agents, antioxidants, mold release agents, flame retardants, lubricants, colorants, flow enhancers, fillers including nanofillers, light and ultraviolet absorbers, pigments, weathering agents, and plasticizers.

Suitable alloys include stainless steel, and alloys of aluminum and titanium.

In embodiments, the bearing cage retainer may be self-lubricating or have a low friction surface to reduce wear. However, this is not necessary in all embodiments, since the parts of the pump where the bearings and the bearing holders are located will normally be oiled.

In an embodiment, the bearing cage retainer is metallic (e.g., stainless steel) with a polymer or inorganic (e.g., ceramic) coating, particularly to form a braking surface. Particularly preferred coatings for forming the braking surface may be selected from the group consisting of: PEEK, polyamide (e.g., nylon), polyoxymethylene, PTFE, and molybdenum disulfide.

Additionally or alternatively, the bearing cage retainer may comprise a thrust bearing, typically a thrust ball bearing or a thrust roller bearing. Typically, the thrust bearing will include an annular race that engages the bearing cage when in a failed configuration.

The bearing cage retainer may be located on the outer race or the inner race, either as a part of a bearing damper, a bearing retainer nut, between a felt cylinder (felt pot) and the pump body, as a part of a lubrication cartridge, as a part of a lubrication stack, or as a part of an oil delivery nut. Preferably, the bearing cage retainer may be refitted (retromated) to the turbomolecular pump during maintenance of the turbomolecular pump.

Typically, the bearing cage retainer does not form part of the bearing, i.e., the bearing cage retainer is not formed integrally with the bearing itself. However, in embodiments, the limiter may form a portion of the inner race and/or the outer race. Such an arrangement is advantageous because it can reduce the number of parts in the tolerance stack and can allow the retainer to move with the damper when in use, allowing for a smaller gap between the cage and the retainer.

The invention also provides a bearing damper, a bearing fixing nut, a lubricating cylinder and/or an oil delivery nut, each of which comprises the bearing retainer limiter according to the invention.

The invention also provides a turbomolecular pump comprising a bearing cage retainer according to any other aspect or embodiment of the invention. A turbomolecular pump may comprise at least one rotor and at least one stator, each comprising a plurality of rotor blades or stator blades. Preferably, when the limiter is in a failed state/configuration, the rotor(s) and stator(s) or vanes thereof are not in contact with each other.

Accordingly, the present invention may provide a turbomolecular pump comprising a bearing cage retainer, and further comprising: a rolling element rotor bearing comprising an outer race, an inner race, and a plurality of rolling elements within a rotatable bearing cage; a rotatable rotor shaft coupled to at least one substantially annular rotor array; and at least one substantially annular stator array adjacent to and operatively spaced from the rotor array; wherein the rotatable rotor shaft is coupled to an inner race of the bearing, and wherein the rotor array and the stator array are maintained in a spaced apart relationship when the bearing cage retainer is in a failed configuration.

Additionally or alternatively, the present invention also provides a turbomolecular pump comprising a rolling element rotor bearing comprising an outer race, an inner race, and a plurality of rolling elements located within a bearing cage, the turbomolecular pump further comprising a bearing cage retainer configured to engage the bearing cage when the bearing cage has become axially displaced relative to the outer race by a predetermined distance in the direction of the bearing cage retainer. Typically, the predetermined distance is no more than half the depth of the rolling elements, for example no more than half the depth of the balls. A predetermined distance of about 50 μm to about 3mm may be used, with 0.5 mm being one example.

Generally, a turbomolecular pump comprises at least one rotor and at least one stator. Preferably, when the limiter and bearing holder are engaged, the rotor(s) and stator(s) or vanes thereof are not in contact.

The invention also provides a rolling element rotor bearing system for a turbomolecular pump. The bearing system may include: a rolling element rotor bearing comprising an outer race, an inner race, and a plurality of rolling elements within a rotatable bearing cage; an axial stop configured to limit longitudinal axial displacement of an outer race of the bearing; and a bearing cage retainer located below the bearing cage and configured such that at a maximum longitudinal axial displacement limit of the outer race in a direction of the retainer, the bearing cage retainer is not engaged with the bearing cage.

The system is arranged such that in a failure configuration, the bearing cage retainer engages the bearing cage and limits axial displacement of the bearing cage relative to the outer race to an extent such that the bearing cage maintains the rolling elements in spaced relation. The failure configuration is characterized by misalignment of the bearing cage due to longitudinal axial displacement of the bearing cage relative to the outer race in the direction of the stop. Preferably, the rolling elements are each held in rolling, sliding engagement with both the inner and outer races of the bearing. Additionally or alternatively, the limiter limits axial displacement of the bearing cage relative to the outer race to an extent such that the bearing cage and preferably the inner race remain rotatable, preferably wherein the axis of rotation of the bearing cage remains substantially coaxial with the axis of rotation of the rotor shaft of the vacuum pump and/or the inner race.

The present invention also provides a method of manufacturing a turbomolecular pump comprising installing a bearing cage retainer as disclosed elsewhere in this application.

Drawings

Preferred features of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

fig. 1 shows a prior art bearing mounted in place in a turbomolecular pump.

Fig. 2 shows a bearing cage retainer according to the present invention.

Fig. 3 shows a bearing cage retainer according to the present invention.

Fig. 4 shows a bearing cage retainer according to the present invention.

Fig. 5 shows a bearing cage retainer according to the present invention.

Fig. 6 shows a bearing cage retainer according to the present invention.

Detailed Description

The invention provides a bearing cage retainer for a rolling element rotor bearing in a turbomolecular pump.

As illustrated in fig. 2 and 6, in an example, the bearing cage retainer (11) is formed integrally with the bearing fixing nut (12). Advantageously, this allows the bearing cage retainer (11) to be introduced during maintenance of the turbomolecular pump (4) by simply replacing the previous bearing fixing nut (10) with a bearing fixing nut (12) according to the invention comprising a bearing cage retainer (11). The nut (12) is typically vented and/or toothed to allow oil to drain from the bearing.

In the failure configuration, the illustrated bearing cage retainer (11) is employed to engage the bearing cage (7). As better illustrated in fig. 6, the bearing cage retainer comprises three pads (17, 18, 19), each pad comprising a braking surface (20, 21, 22). In the failure configuration, it is the braking surfaces (20, 21, 22) that engage the bearing cage. The illustrated bearing retainer nut is a single, metallic, unitary structure; however, the braking surface may be in the form of a coating, such as a polymer or ceramic coating. As shown in fig. 2, in the illustrated operating arrangement, the braking surface (20) lies in a plane that intersects both the inner race (2) and the outer race (5), and that plane is substantially tangential to the axis (a) of the impeller shaft.

The rolling element rotor bearing illustrated in fig. 2 is a ball bearing. The bearing comprises an inner race (2) fixedly attached to an impeller shaft (3) of the vacuum pump. In use, the impeller shaft (3) rotates about an axis (a), wherein the arrow indicates an upward direction during normal use. The axis of rotation of the inner race (2) and the bearing cage is substantially coaxial with the axis (a). The illustrated bearing comprises a series of balls (6), typically about 6 to 12, located in a bearing cage (7). In normal use, the bearing cage (7) maintains a circumferentially uniform spacing of the balls (6). Typically, maintaining a substantially uniform spacing of the balls maintains the bearing cage and inner race in substantially coaxial alignment with axis a. The bearing cage (7) may for example be of the snap-fit type Torlon toothed rings. The illustrated ball bearing (1) and bearing retainer nut (12) with integrally formed bearing cage retainer (11) form a rolling element rotor bearing system (23) for a turbomolecular pump.

In fig. 2, the outer race (5) is fixedly attached to the bearing support damper (8). Typically, the outer race (5) is attached to the support damper (8) using an adhesive.

In fig. 2, the bearing cage retainer (11) is spaced from the bearing cage an axial distance sufficient such that the bearing cage retainer (11) will not engage the bearing cage (7) unless the bearing cage (7) has become axially displaced (i.e., misaligned) downward relative to the outer race (5). Fig. 2 shows the operational arrangement for the bearing cage retainer (11).

In use, the bearing retainer nuts (10, 12) provide an axial stop (backstop) for the bearing support damper (8). Typically, the bearing retainer nuts (10, 12) prevent downward axial displacement of the bearings by more than 150 μm. Thus, the bearing cage retainer (11) may be arranged such that it is spaced 50 μm or more in the axial direction from the bearing cage (7) at its most downward operating position. Typically, the spacing is about 50 μm to about 3mm, with 0.5 mm being one example. Preferably, the spacing is no more than half the depth of the rolling elements, for example no more than half the depth of the balls.

In the failure configuration, the bearing cage (7) may become axially displaced downward relative to the outer race (5) and contact the bearing cage retainer (11). This may occur due to contamination in the bearing (1) causing the bearing cage (7) to come out of the bearing (1). In such a failure configuration, the bearing cage retainer (11) prevents the bearing cage (7) from completely backing out of the bearing (1) and maintains the circumferential spacing of the bearing balls (6), preferably the circumferential uniform spacing of the bearing balls (6). By maintaining the spacing of the bearing balls (6), the axial alignment of the impeller shaft can be maintained, preventing the rotor of the pump from contacting its stator and thereby avoiding catastrophic failure. If the vacuum pump (4) is running at the time of reaching the failure configuration, an audible noise or vibration change will alert the user, thereby enabling the pump (4) to stop in a controlled manner. Thus, the bearing retainer (11) enables the bearing (1) to fail safely. The bearing (1) will fail and will need to be replaced, typically together with the bearing cage retainer (11); however, the vacuum pump (4) and the user's instruments (not shown) may be largely unaffected by the failure.

Fig. 3 illustrates an alternative embodiment of the present invention. In this embodiment, a separate bearing cage retainer (11) is located between the felt barrel (13) and the pump body (14). As with other embodiments of the invention, in the illustrated operating arrangement, the bearing cage retainer (11) is axially spaced from the bearing cage (7) by a distance sufficient such that the bearing cage retainer (11) will not engage the bearing cage (7) unless the bearing cage (7) has become axially displaced downwardly relative to the outer race (5) (e.g., in a failure configuration).

Also, the bearing cage retainer (11) is preferably apertured and/or toothed to enable lubricant to flow around the bearing.

Advantageously, the bearing cage retainer (11) can be inserted when the felt cartridge (14) is replaced during routine maintenance of the vacuum pump (4).

Fig. 4 illustrates yet another example of the present invention, in which the bearing cage retainer (11) forms part of the lubrication barrel (15). As with other embodiments of the invention, the illustrated bearing cage retainer (11) is axially spaced from the bearing cage (7) by a distance sufficient that the bearing cage retainer (11) will not engage the bearing cage unless the bearing cage (7) has become axially displaced downward relative to the outer race (5).

Also, advantageously, the bearing cage retainer (11) may be installed when the lubrication cartridge (15) is replaced during servicing.

Fig. 5 illustrates yet another example of the present invention, in which the bearing cage retainer (11) is formed integrally with the oil delivery nut (16). The illustrated bearing cage retainer also functions as an oil slinger, which may be advantageous.

A bearing cage retainer (11) is coupled to the impeller shaft (3). Thus, when the pump is in use, the bearing cage retainer (11) rotates about the axis (a) of the impeller, albeit at a different speed than the bearing cage (7). The moving bearing cage retainer (11) helps to minimize wear on the cage (7) as it is removed. The skilled person will be able to select whether the stopper (11) is static or rotating is most advantageous for a particular bearing (1) and/or vacuum pump (4).

It will be understood that various modifications may be made to the illustrated embodiments without departing from the spirit and scope of the invention as defined by the appended claims as interpreted according to the patent laws.

Claims

1. A bearing cage retainer for a rolling element rotor bearing in a vacuum pump; the bearing includes an outer race, an inner race, and a plurality of spaced rolling elements held within a rotatable bearing cage; the vacuum pump is configured to limit longitudinal axial displacement of an outer race of the bearing; and the bearing cage retainer is configured with: an operating arrangement in which the bearing cage retainer is not engaged with the bearing cage at a maximum longitudinal axial displacement limit of the outer race in the direction of the retainer; and a failure configuration characterized by misalignment of the bearing cage due to longitudinal axial displacement of the bearing cage relative to the outer race in the direction of the retainer, and in which the bearing cage retainer engages the bearing cage and the bearing cage maintains the rolling elements spaced within the bearing.

2. The bearing cage retainer of claim 1, wherein in the failed configuration, the bearing cage retainer supports the bearing cage such that each of the rolling elements is in rolling and/or sliding engagement with both the outer race and the inner race of the bearing.

3. The bearing cage retainer according to claim 1 or 2, wherein the bearing cage retainer is configured such that the bearing cage is rotatable when the bearing cage retainer is in the failure configuration, preferably wherein the axis of rotation of the bearing cage is substantially coaxial with the axis of rotation of the rotor shaft of the vacuum pump and/or the inner race.

4. The bearing cage retainer of any preceding claim, wherein upon transition from the operational arrangement to the failure configuration as the bearing rotates, the bearing cage retainer provides an audible signal and/or a detectable change in vibration indicative of the bearing failure.

5. The bearing cage retainer of any preceding claim, comprising at least one bearing cage braking surface for frictional sliding engagement with the bearing cage in the failure configuration.

6. The bearing cage retainer of claim 5, wherein the at least one braking surface is substantially annular or partially annular.

7. The bearing cage retainer according to claim 5 or 6, wherein a ratio of an extent of the braking surface in the radial direction to an extent of the bearing cage in the radial direction is about 1:1 to about 1: 2.

8. The bearing cage of any of claims 5 to 7, wherein in the failure configuration the at least one braking surface engages only the bearing cage.

9. A bearing cage according to any of claims 5 to 8, wherein in the operating arrangement the at least one braking surface lies in a plane which intersects the outer race and/or the inner race of the bearing, preferably the plane is substantially tangential to the axis of rotation of the bearing.

10. The bearing cage according to any of claims 5 to 9 comprising a part annular boss, wherein the at least one braking surface is a bearing cage side surface of the boss.

11. The bearing cage retainer of any of claims 1 to 4, comprising a thrust race bearing configured to engage the bearing cage when in the failed configuration.

12. The bearing cage retainer of any preceding claim, wherein the rolling elements are balls and in the failure configuration at least a hemisphere of each of the balls remains located between the inner and outer races.

13. A turbomolecular pump comprising a bearing cage retainer according to any preceding claim, and further comprising: a rolling element rotor bearing comprising an outer race, an inner race, and a plurality of rolling elements within a rotatable bearing cage; a rotatable rotor shaft coupled to at least one substantially annular rotor array; and at least one substantially annular stator array adjacent to and operatively spaced from the rotor array; wherein the rotatable rotor shaft is coupled to the inner race of the bearing, and wherein the rotor array and stator array are spaced apart when the bearing cage retainer is in a failed configuration.

14. A bearing retainer nut for a bearing damper of a turbomolecular pump, the bearing nut comprising a bearing cage retainer according to any of claims 1 to 12.

15. A rolling element rotor bearing system for a turbomolecular pump, the bearing system comprising:

a rolling element rotor bearing comprising an outer race, an inner race, and a plurality of spaced rolling elements held within a rotatable bearing cage;

an axial stop configured to limit longitudinal axial displacement of the outer race of the bearing; and

a bearing cage retainer located below the bearing cage and configured such that at a maximum longitudinal axial displacement limit of the outer race in the direction of the retainer, the bearing cage retainer is not engaged with the bearing cage; and is

Wherein, in a failure configuration characterized by misalignment of the bearing cage due to longitudinal axial displacement of the bearing cage relative to the outer race in the direction of the retainer, the bearing cage retainer engages the bearing cage and limits axial displacement of the bearing cage relative to the outer race to an extent such that the bearing cage maintains the rolling elements spaced within the bearing.